1. Field of the Invention
The present invention relates to an electronic component built-in module that houses electronic components including, for example, an active component such as a transistor or the like and a passive component such as a capacitor or the like, and a method of manufacturing the electronic component built-in module.
2. Related Background Art
In recent years, it has been requested that electronic equipment be capable of high performance and reduced in size. This has led to a demand for higher-density mounting and higher performance of electronic components. With this as a background, there also has been a demand for a small-sized circuit substrate on which electronic components can be mounted with high density. With respect to these demands, as a circuit substrate that allows high-density mounting, a multilayer circuit substrate in which wires formed on different layers are connected electrically by an inner via has been under development. By the use of a multilayer circuit substrate, wire connections between electronic components can be shortened, thereby allowing high-density wiring to be realized. However, even in a multilayer circuit substrate that allows high-density wiring as described above, electronic components mounted on a surface of the circuit substrate still constitute a large portion of the multilayer circuit substrate, which has been a hindrance to a reduction in size.
With respect to this problem, a technique has been proposed that realizes high-density mounting of electronic components by embedding the electronic components in a circuit substrate. However, in the case of a circuit substrate formed from ceramics, a firing process is required when housing electronic components, and thus an electronic component containing a semiconductor such as silicon or the like cannot be housed, which has been disadvantageous. As a solution to this, a circuit substrate also has been proposed that includes an insulating layer formed from a resin so that electronic components are housed therein (see, for example, JP 3-69191 A and JP11-103147 A). With this configuration, manufacturing can be performed at a relatively low temperature, and thus even an electronic component containing a semiconductor can be housed. However, due to the low thermal conductivity of the resin, it is difficult to radiate heat generated from the electronic components. Moreover, in the case where the electronic components are mounted with high density, heat radiation further is hindered. Further, inevitably, an influence also is exerted by the generation and propagation of noise due to a high-speed operation and high-density mounting.
As described above, in an electronic component built-in module in which electronic components are housed in a circuit substrate (mounted three-dimensionally in the circuit substrate), it is required to achieve not only high-density mounting and a high-level function but also an effect of reducing noise and an excellent heat radiation property. As a measure to meet this requirement, an electronic component built-in module has been proposed that includes an insulating layer formed from a mixture (composite resin material) containing a thermosetting resin such as an epoxy resin or the like and an inorganic filler such as ceramic powder or the like so that electronic components are embedded therein (see, for example, JP11-220262 A).
FIG. 10 shows an example of an electronic component built-in module in which electronic components are embedded in an insulating layer formed using a mixture of a thermosetting resin and an inorganic filler. In this electronic component built-in module 100, circuit substrates 101 on which electronic components (an active component 104a and a passive component 104b) are mounted are joined respectively to each main face of an insulating layer 102, and the electronic components 104a and 104b are embedded in the insulating layer 102. Moreover, wiring patterns 107 formed on the different circuit substrates 101 are connected electrically by an inner via 103 formed from a conductive resin composition. The insulating layer 102, which is formed from a mixture of an inorganic filler and a thermosetting resin, can be controlled in terms of a thermal conductivity, a dielectric constant and the like by selecting the type of the inorganic filler. Thus, an electronic component built-in module that is excellent in the effect of reducing noise and heat radiation property can be manufactured.
In an electronic component built-in module having the above-mentioned configuration, the reliability of an inner via is of importance. For example, due to heat applied when another electronic component is mounted by soldering on the completed electronic component built-in module, heat generated during an operation of equipment, a change in the temperature of an environment of use and the like, an insulating layer undergoes repeated cycles of thermal expansion and contraction. In this case, because of a difference in coefficient of thermal expansion between the insulating layer and a conductive resin composition constituting an inner via, a stress is generated in the inner via. Such a stress causes a deterioration in the connection reliability of an inner via. The insulating layer is formed so as to have a thickness not smaller than the height of each electronic component to be housed, and thus necessarily, the inner via penetrating the insulating layer in a thickness direction also has an increased length. Further, in order to achieve higher mounting density, it also is required that the inner via be decreased in diameter, and thus the inner via tends to have a high aspect ratio. Therefore, due to repeated cycles of the thermal expansion and contraction of the insulating layer in the thickness direction, a particularly large influence is exerted on a deterioration in the connection reliability of an inner via. In the case where a coefficient of thermal expansion of the insulating layer is larger than a coefficient of thermal expansion of circuit substrates, the expansion of the insulating layer in a plane direction is constrained by the circuit substrates, so that the insulating layer tends to expand in the thickness direction. Because of this, the connection reliability of an inner via becomes more likely to be deteriorated.